Multi-Pathlength Mid-IR Spectroscopy of Proteins in Aqueous Solutions Using an External Cavity Quantum Cascade Laser

Area of Science:

• Analytical Chemistry
• Spectroscopy
• Biophysics

Background:

• Mid-infrared (mid-IR) spectroscopy is valuable for molecular analysis but faces challenges in aqueous systems due to water absorption and limited detector dynamic range.
• These limitations restrict optical pathlength, hindering direct analysis of complex liquid samples without dilution.

Purpose of the Study:

• To develop a novel quantum cascade laser-based mid-IR spectroscopic setup for direct analysis of aqueous samples.
• To overcome limitations of detector dynamic range and solvent absorption in mid-IR spectroscopy.
• To establish a method for determining optimal optical pathlengths for complex samples.

Main Methods:

• Utilized a quantum cascade laser source and a custom wedge-shaped flow cell for simultaneous multi-pathlength measurements.
• Employed a pyroelectric detector array for data acquisition.
• Introduced a pathlength-to-noise ratio metric for data integration and optimal pathlength determination.
• Validated the system with aqueous solutions of bovine serum albumin and β-lactoglobulin.

Main Results:

• Successfully acquired mid-IR absorption spectra across a continuum of optical pathlengths without mechanical adjustments or sample dilution.
• Resolved characteristic amide I and II bands in protein solutions.
• Detected differences in protein secondary structures.
• Demonstrated robust integration of multi-pathlength data into a single spectrum.

Conclusions:

• The developed spectroscopic setup offers a flexible approach to mid-IR absorption spectroscopy for aqueous systems.
• The method effectively addresses detector dynamic range limitations and enhances understanding of optimal pathlength selection.
• Enables direct, sensitive analysis of complex biological liquids like protein solutions.